Borehole plugs are used in a variety of applications for zone isolation. In some applications the differential pressure experienced in the set position can come from opposed directions. These plug typically have a sealing element with mirror image slips above and below the sealing element. The plug is set with a setting tool that creates relative movement between a setting sleeve that is outside the mandrel and the plug mandrel. The slips have wickers oriented in opposed directions and ride out on cones to the surrounding tubular. The sealing element is axially compressed after the first set of slips bite followed by setting of the other set of slips on the opposite side of the sealing element from the first slip set to set. The set position of these elements is maintained by a body lock ring assembly. Body lock ring assemblies are in essence a ratchet device that allows relative movement in one direction and prevents relative movement in the opposite direction. The relative movement that compresses the sealing element and drives the opposed slips out on respective cones is locked by a body lock ring. Body lock rings are threaded inside and out and sit between two relatively movable components. The thread forms are such that ratcheting in one direction only is enabled. A good view of such a design is shown in FIG. 13 of U.S. Pat. No. 7,080,693. The trouble with such a design in applications where the plug needs to be quickly milled out after use such as in treating or fracturing is that the shear loading on the ratcheting patterns is so high that the ratchet teeth break at loads that are well within the needed operating pressure range for the plug. With fracturing pressures going up and the use of readily milled components such as composites a new approach to locking was needed. The goal during treating is to hold the differential pressure from above while keeping the design simple so as not to prolong the milling time for ultimate removal. A typical zone treatment can involve multiple plugs that need to be removed. Elimination of upper slips when using the lock ring of the present invention also shortens milling time. Better yet, milling of the plugs can be avoided by lowering pressure from above to induce flow back from the stage below the targeted plug, until the slips of the plug or series of plugs to disengage and come up to a surface location such as into specialized surface or subsurface equipment where the pressure can be relieved and the plug or plugs safely removed.
The lock ring is preferably split to ease its movement when axial opposed forces are applied to set the plug. The ring is tapered in cross section to allow it to act as a wedge against reaction force tending to relax the components from the set position. The side of the ring facing the mandrel has a surface treatment that provides minimal resistance in the setting direction and digs into the mandrel to resist reaction forces from the compressed sealing element in the set position. Preferably the surface treatment is a series of extending members oriented downhole with sharp ends that can dig into the mandrel for a firm grip. These and other aspects of the present invention can be better understood by those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.
Multicomponent body lock rings have been made of easily milled materials such as composites as illustrated in US 2014/0190685; U.S. Pat. Nos. 8,191,633; 6,167,963; 7,036,602; 8,002,030 and 7,389,823. The present invention presents a way to avoid milling altogether so that the use of composites that aid milling become an optional feature. This can reduce the cost of each plug in treatments that frequently involve multiple plugs. U.S. Pat. No. 8,240,390 is relevant to packer releasing methods.